The present disclosure relates to an optical recording medium and a recording device thereof, and particularly one that is suitable for being applied in a case when tracking servo with respect to recording light is performed by an adjacent track servo.
As optical recording media in which recording and reproduction of a signal is performed by irradiation of light, so-called optical discs, for example, CDs (Compact Discs), DVDs (Digital Versatile Discs), BDs (Blu-ray Discs: registered trademark), and the like have been popularized.
In relation to an optical recording medium that is expected to become the next generation of optical recording media that are currently popularized such as CDs, DVDs, and BDs, the present applicant has already proposed a so-called bulk recording type optical recording medium that is described in Japanese Unexamined Patent Application Publication No. 2008-135144 and Japanese Unexamined Patent Application Publication No. 2008-176902.
Here, bulk recording is a technique of increasing recording capacity by performing multi-layer recording within a bulk layer 102 by performing laser light irradiation while successively changing the focal position with respect to an optical recording medium (bulk type recording medium 100) that includes at least a cover layer 101 and the bulk layer (recording layer) 102 as illustrated in FIG. 9, for example.
In relation to such bulk recording, a recording technique known as a micro-holographic method is disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144.
With the micro-holographic method, a so-called holographic recording material is used as the recording material of the bulk layer 102. As a holographic recording material, for example, a photopolymerization type photopolymer or the like is widely used.
The micro-holographic method is broadly divided into a positive type micro-holographic method and a negative type micro-holographic method.
The positive type micro-holographic method is a technique of forming minute interference patterns (holograms) by concentrating light of two opposing luminous fluxes (luminous flux A, luminous flux B) in the same position, which become the recording marks.
Further, the negative type micro-holographic method is the opposite idea to the positive micro-holographic method, and is a technique in which interference patterns that are formed in advance are erased and the erased portions become the recording marks. With the negative type micro-holographic method, an initialization process for forming interference patterns with respect to the bulk layer 102 in advance before performing the recording action is performed. Specifically, as the initialization process, luminous fluxes by parallel light are irradiated to be opposing, and the interference patterns are formed over the entirety of the bulk layer 102.
Further, having formed the interference patterns in advance by the initialization process in such a way, information recording by the formation of erasure marks is performed. That is, information recording by erasure marks is performed by performing laser light irradiation according to the recording information in a state in which the focus is on an arbitrary layer position.
Furthermore, the applicant also proposes a recording technique of forming, for example, voids (vacancies) as disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902 as recording marks as a technique of bulk recording which is different from the micro-holographic method.
The void recording method is a technique of recording vacancies (voids) within the bulk layer 102 by performing laser light irradiation at relatively high power on a bulk layer 102 that is configured, for example, by a recording material such as a photopolymerization type photopolymer. As described in Japanese Unexamined Patent Application Publication No. 2008-176902, the vacancy portions that are so formed become portions with a different refraction index from other portions within the bulk layer 102, and the reflection rate of light in bordering portions therebetween is increased. The vacancy portions therefore function as recording marks, and information recording by the formation of the vacancy marks is thereby realized.
Since such a void recording method does not form holograms, light irradiation from only one side is sufficient. That is, recording marks are not formed by concentrating light of two luminous fluxes on the same position as in the case of the positive type micro-holographic method.
Further, in a comparison with the negative micro-holographic method, there is an advantage that an initialization process is not performed.
Here, although an example in which irradiation of pre-curing light before recording is performed in performing void recording is shown in Japanese Unexamined Patent Application Publication No. 2008-176902, recording of voids is possible even if the irradiation of such pre-curing light is omitted.
Here, although the optical recording medium is a bulk recording type (also referred to simply as bulk type) optical recording medium in which various types of recording techniques above are proposed, the recording layer (bulk layer) of an optical recording medium of such a bulk type does not explicitly include a multilayer structure whereby a plurality of reflecting films, for example, are formed. That is, position guides for each recording layer as included in an ordinary multilayer disc or reflective films on which such guides are formed are not provided in the bulk layer 102.
Therefore, with the structure of the bulk type recording medium 100 as illustrated in FIG. 9 earlier, it is not possible to perform focus servo or tracking servo when recording when marks are not formed.
For such a reason, with respect to the bulk type recording medium 100, reflecting surfaces (reference surfaces) with position guides as illustrated in FIG. 10 are provided as references.
Specifically, position guides (guide channels) formed as pits and grooves, for example, are formed on the lower surface side of a cover layer 101 in a spiral form or a concentric form, and a selective reflection film 103 is formed thereon. The bulk layer 102 is then formed on the lower layer side of the cover layer 101 on which the selection reflection film 103 is formed in such a manner via an adhesive material such as, for example, a UV curing resin as an intermediate layer 104 in the drawing.
Here, recording of the absolute position information (address information) such as, for example, the radial position information or the rotation angle information is performed by the formation of the guide channels described above such as pits and grooves. In the description below, a surface on which such guide channels are formed and recording of absolute position information is performed (here, the formation surface of the selective reflection film 103) is referred to as a “reference surface Ref”.
In addition, on top of the medium structure as described above, servo laser light (also referred to simply as servo light) as laser light for position control is irradiated on the bulk type recording medium 100 separately from the laser light for the recording of marks (hereinafter, also referred to as recording laser light or simply recording light) as illustrated in FIG. 11 below.
As illustrated in the drawings, such recording laser light and servo laser light are irradiated on the bulk type recording medium 100 via a common objective lens.
At this time, there is a concern that if the servo laser light reaches the bulk layer 102, there may be a detrimental effect on the mark recording within the bulk layer 102. In the bulk recording method of the related art, therefore, laser light with a different wavelength band from the recording laser light is used as the servo laser light, and the selective reflection film 103 with a wavelength selectivity that reflects servo laser light and transmits recording laser light is provided as the reflection film that is formed on the reference surface Ref.
The action of recording marks on the bulk type recording medium 100 will be described with reference to FIG. 11 upon the premise described above.
First, when performing multilayer recording on the bulk layer 102 on which guide channels or reflection films are not formed, where the positions of the layer positions on which marks are recorded in the depth direction within the bulk layer 102 are is determined in advance. In the drawing, a case is exemplified in which a total of five information recording layer positions L from a first information recording layer position L1 to a fifth information recording layer position L5 are set as layer positions on which marks are formed within the bulk layer 102 (mark formation layer position: also referred to as information recording layer position). As illustrated in the drawings, the first information recording layer position L1 is set at a position that is separated from the selective reflection film 103 (reference surface Ref) on which guide channels are formed by a first offset of-L1 in the focus direction (depth direction). Further, the second information recording position L2, the third information recording layer position L3, the fourth information recording layer position L4, and the fifth information recording layer position L5 are respectively set in positions that are separated by a second offset of-L2, a third offset of-L3, a fourth offset of-L4, and a fifth offset of-L5.
When recording while marks are not yet formed, it is not possible to perform focus servo or tracking servo with each of the layer positions within the bulk layer 102 as the targets based on the reflected light of the recording laser light. Therefore, focus servo control and tracking servo control of the objective lens when recording is performed by spot positions of the servo laser light following the guide channels on the reference surface Ref based on the reflected light of the servo laser light.
However, it is important that the recording laser light reaches the bulk layer 102 that is formed on a lower layer side than the reference surface Ref for mark recording. Therefore, in such an optical system, a recording light focusing mechanism for independently adjusting the focus positions of the recording laser light are provided separately from the focusing mechanism of the objective lens.
Specifically, an expander that changes the collimation state of the recording laser light (dispersed, parallel, converged) that is incident on the objective lens is provided as such a recording light focusing mechanism. That is, by changing the collimation state of the recording laser light that is incident on the objective in such a manner, the focus positions of the recording laser light are able to be adjusted independently from the servo laser light.
By providing such a focusing mechanism for the recording laser light, by the focus and tracking servo control of the objective lens being performed based on the reflected light of the servo laser light from the reference surface Ref as described above, the focus positions of the recording laser light match the desired information recording layer positions L within the bulk layer 102 while being controlled to positions that correspond to the guide channels that are formed on the reference surface Ref in the tracking direction.
Here, when performing reproduction of the bulk type recording medium 100 on which mark recording is already performed, the position of the objective lens may not be controlled based on the reflected light of the servo laser light as when recording. That is, when reproducing, focus and tracking servo control of the objective lens may be performed based on the reflected light of the laser light that is irradiated with mark rows that are formed on information recording layer positions L (also referred to as information recording layers L when reproducing) that are the reproduction targets as targets.
As described above, with the bulk recording method, while irradiating recording laser light for performing mark recording and servo light as position control light on the bulk type recording medium 100 via a common objective lens (by synthesizing over the same optical axis), when recording, by performing focus servo control and tracking servo control of the objective lens such that the servo laser light follows the position guides on the reference surface Ref and separately adjusting the focus positions of the recording laser light by the recording light focusing mechanism described above, even if position guides are not formed within the bulk layer 102, mark recording is possible in the desired positions (depth direction and tracking direction) within the bulk layer 102.
Furthermore, when reproducing, it is possible to perform focus servo control and tracking servo control of the objective lens based on the reflected light of the light that is irradiated with the mark rows that are already recorded as targets. That is, servo control by the servo laser light when reproducing is unnecessary.
Here, in a case when the bulk recording method that has hitherto been described is adopted, spot position deviation in the recording surface direction of the recording laser light and the servo laser light occurs accompanying the generation of skew (tilt) or the generation of lens shift of the objective lens that accompanies disc eccentricity.
That is, a change in the relative positional relationship of the spot position of the recording laser light and the spot position of the servo laser light in the recording surface direction (tracking direction) occurs accompanying the generation of skew or lens shift, and as a result, it becomes not possible to perform mark recording on intended positions within the bulk layer 102.
Such spot position deviation is caused by positions of the spot positions of the recording laser light and the spot positions of the servo laser light in the recording surface direction being designed to match in an ideal state in which there is no skew or lens shift in the optical system of the bulk recording system described above.
If deviation in the information recording positions occurs accompanying such skew and lens shift, there is a concern that information recording positions overlap between adjacent tracks particularly in a case when additional recording that is accompanied by disc replacement is performed.
Specifically, since eccentricity or skewing of a disc may occur in different forms when the disc is loaded according to the clamping of the disc on the spindle motor or the like, when performing additional recording that is accompanied by disc replacement such as, for example, performing additional recording by loading a disc on a different drive after performing recording on a given disc with a given drive, there is a concern that overlapping or in some cases crossing of the mark rows of the recording portions and the mark rows of the additional recording portions occurs due to the form of the skewing and the eccentricity when recording previously and the form of the skewing and the eccentricity when recording additionally being different.
If overlapping or crossing of mark rows occurs in such a way, it becomes no longer possible to correctly reproduce recording signals.
Therefore, as one technique for preventing the occurrence of such overlapping or crossing, setting the track pitch of the reference surface Ref to be relatively large is exemplified.
However, in a case when the track pitch of the reference surface Ref is enlarged, naturally, shrinking of the recording capacity of the bulk layer 102 is caused.
Further, as another technique, for example, a system in which there is no replacement of a disc such as a hard disk is considered.
However, since exchanging of discs is naturally not performed at all with such a technique, when there is a problem with the disc, for example, it is not possible to exchange only the disc. Further, it is not possible to read data that is recorded by a given recording device with another recording device. The result, therefore, is that convenience is lost in such respects.
As an effective technique for avoiding such problems, therefore, adopting a so-called ATS (Adjacent Track Servo) technique is considered. The ATS was originally considered as a self servo track writer (SSTW) for a hard disk drive.
FIG. 12 is a diagram for describing the ATS.
As shown in the drawing, in the ATS, a recording spot S_rec and an adjacent track servo spot S_ats are formed on a recording medium (within the bulk layer 102). The recording spot S_rec and the adjacent track servo spot S_ats are formed by irradiating light beams that are respectively the origins thereof via a common objective lens. At this time, the distance between each of the spots S is fixed.
With the ATS, the recording spot S_rec is the leading spot (that is, the outer circumference side in a case when the progress direction of the recording is from the inner circumference to the outer circumference), the adjacent track servo spot S_ats is the following spot, and tracking servo is performed by the adjacent track servo spot S_ats with the mark rows that are formed by the recording spot S_rec as the targets. That is, tracking servo control of the objective lens is performed such that the adjacent track servo spot S_ats follows one track before the recording spot S_rec is formed.
According to such an ATS, since the track pitch is fixed as the distance between each of the spots S, the problem whereby the tracks overlap (information recording positions overlapping) due to the influence of eccentricity or the like does not occur. That is, there is no reason to unnecessarily enlarge the track pitch in consideration of the deviation of recording positions due to eccentricity or the like as described above, or to provide a system in which a disc is not removable.